Polyalkylene terephthalates have acquired considerable significance as starting materials for the production of fibers, films and shaped articles. By virtue of their partially crystalline structure, they show outstanding properties, for example, high wear resistance, favorable creep rupture strength and high dimensional stability, and are therefore particularly suitable for the production of shaped articles subjected to heavy mechanical and thermal stressing. An additional improvement in their mechanical properties can be obtained by the incorporation of reinforcing materials, for example glass fibers (U.K. patent specification No. 1,111,012, U.S. Pat. No. 3,368,955, German Auslegeschrift No. 2,042,447).
By virtue of its special physical properties, polyethylene terephthalate (PET) is particularly suitable for the production of fiber products and films, but is scarcely suitable for injection-molding because high mold temperatures (about 140.degree. C.) and relatively long molding times are required for this purpose. This serious disadvantage almost completely prevents the use of polyethylene terephthalate for injection-molding despite its high rigidity and thermal stability.
Although polypropylene terephthalate (PPT) and polybutylene terephthalate (PBT) require shorter molding times and lower mold temperatures (about 100.degree. C.) by virtue of their higher crystallization rate, they show poorer physical properties, in particular less thermal stability than polyethylene terephthalate.
It has also been tried to produce polycondensates which combine the favorable properties both of polyethylene terephthalate and of polypropylene or polybutylene terephthalate. For example, it is known that the tendency towards crystallization of polyethylene terephthalate can be improved by nucleation with finely divided, solid inorganic materials (Netherlands patent specification No. 6,511,744).
High crystallinity guarantees hardness and dimensional stability, even at elevated temperatures. This high crystallinity should be reached as quickly as possible in order to ensure that optimum properties are obtained. In addition, the residence time in the mold determines the injection cycle, the length of which is one of the determining factors of the economy of the process. Even at high mold temperatures, these cycles are too long and, for this reason, are an obstacle to the acceptance of polyethylene terephthalate for use in the production of shaped articles by injection molding.